An electrode-arc model, and a three dimensional weld pool model considering the self-adaptive heat source, the shear stress caused by plasma flow, and the deformation of the top and bottom surfaces, are developed to investigate the weld pool convection and keyhole formation mechanism in the keyhole plasma arc welding. With the help of the X-ray transmission system, high-speed video camera and thermal camera, the fluid flow and the temperature distribution of the weld pool are studied. The numerical and experimental results show that: after the penetrated keyhole formation, two convective eddies are formed inside the weld pool. The molten metal flows backward at the top surface of the weld pool, and flows downward and backward at the bottom surface of the weld pool. The low temperature distribution in the weld pool is attributed to the strong fluid flow and energy transportation caused by the much high arc pressure and plasma shear stress. Both the arc pressure driven weld pool deformation and shear stress driven weld pool deformation are responsible for the keyhole formation, and the first mechanism is especially dominant. (C) 2018 Elsevier Ltd. All rights reserved.